1. Field of the Invention
This invention relates to dc-SQUID magnetometry and superconducting electronics and, in particular, to a magnetometer including a superconducting SQUID having an inherent phase shift without application of external magnetic fields.
2. Discussion of Related Art
Very precise measurements of small magnetic fields can be accomplished with a dc-SQUID magnetometer device. A conventional dc-SQUID magnetometer includes a superconducting loop containing a plurality of Josephson junctions, coupled to terminals. Any change in the magnetic field which penetrates the superconducting loop disturbs the current through the device, which is detectable at the terminals. Thus, the dc-SQUID can be used as a device for measuring changes in a magnetic field.
Conventional dc-SQUID magnetometers lack inherent sensitivity. Furthermore, a conventional dc-SQUID magnetometer can only determine the magnitude of the change in a magnetic field, but cannot distinguish the direction of the change. In order to hyper-sensitize a standard dc-SQUID, flux-biasing can be used to shift the latent flux position in the SQUID loop into a linear response regime. A standard dc-SQUID loop behaves in accordance with a well defined current-phase relationship. The equilibrium position of the current-phase relation of a standard dc-SQUID lies in a region of sensitivity where the induced superconducting current is proportional to a small perturbation in the flux squared (I∝Φ2), and resultingly there is no directional sensitivity. By biasing the SQUID loop with an applied flux, the equilibrium position can be shifted into a more sensitive linear response regime, thus introducing directional sensitivity into the current response as well. This can be accomplished by introducing a phase shift of the equilibrium position in the current-phase relation. The phase shift is realized in conventional dc-SQUID devices by application of an external magnetic field to the dc-SQUID device, a technique called flux biasing. In other words, with an externally applied magnetic flux on the SQUID device, a small perturbation in the flux induced by the magnetic field that is being measured will result in a linear response in the superconducting current from the SQUID device.
Furthermore, by coupling multiple SQUID loops, it is possible to enhance the sensitivity of the dc-SQUID magnetometer. See U.S. Pat. No. 5,767,043, entitled “Multiple Squid Direct Signal Injection Device Formed On a Single Layer Substrate,” to Cantor et al., herein incorporated by reference in its entirety. One application of dc-SQUID magnetometry is as a non-destructive testing device in the field of semi-conductor electronics. In the electronics industry, each circuit that is manufactured must be non-destructively tested for correct operating parameters. This is accomplished by running current through the circuit to be tested and measuring the resulting magnetic fields. However, in order to detect flaws in the magnetic field a high degree of resolution is required, which cannot be achieved without flux biasing or coupling the dc-SQUIDs that make up the magnetometer. Thus, there is a necessity for increasing the latent sensitivity of the SQUID magnetometer.
Further applications for dc-SQUID magnetometers range in practical uses. For example, dc-SQUID magnetometers are used in Magnetic Resonance Imaging, microscopic metal defect detection, mine detection, and submarine detection. Additional examples of uses for dc-SQUID magnetometers include analogue-to-digital converters and optical switches. Given the broad range of applications of dc-SQUID Magnetometers, there is a need for devices with increased sensitivity, including directional sensitivity, wherein the overall size and cost of a device is reduced.
There are, however, practical limitations to current methods of dc-SQUID sensitizing. Biasing the loop introduces magnetic fields that may interfere with the fields or system being measured. Similarly, coupling dc-SQUIDs can lead to bulky measurement tools that increase the obtainable distance from the sample, thereby also decreasing the ability to measure magnetic fields in the sample.
The use of a phase shifter in order to sensitize the current-phase behavior in a superconducting loop is known; however, the inherent sensitization has been restricted to a π-phase shift. Thus there is a need for a device that can be used in dc-SQUID magnetometry with a high level of latent sensitivity, as well as directional sensitivity without the application of external magnetic fields.